The use of powder metallurgy (P/M), and particularly iron and iron alloy powders, is known for forming magnets, including soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression molding, injection molding and sintering techniques, can be used to form intricate molded part configurations without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for use immediately after the forming operation.
To date, virtually all powder metal cores for AC electromagnetic applications have been formed of compacted particles of pure iron. As used herein, pure iron is defined as iron with only incidental impurities. As known in the art, pure iron is a soft magnet material that exhibits good magnetic properties and, being highly compressible (i.e., relatively soft and deformable), can be used in powder form to mold parts with reasonably high densities. For example, with the use of appropriate lubricants and/or binders, densities of 98% of theoretical can be achieved. However, many applications for magnets would benefit if a ferromagnetic material of better magnetic properties were used. Examples of such materials include soft magnet materials such as iron alloys, nickel and its alloys, cobalt and its alloys, iron-silicon alloys, iron-phosphorus alloys, iron-silicon-aluminum alloys, ferrites and magnetic stainless steel alloys. In addition, permanent ("hard") magnet materials that might be used include ferrites, iron-rare earth metal alloys, samarium alloys, and ceramic materials. As understood in the art, the terms "soft magnet" and "hard magnet" do not designate the physical hardness of a material, but its relative coercive field strength, with hard magnet materials being capable of exhibiting a very high coercive force that is retained after the magnetizing force is withdrawn. In terms of physical hardness, all of these materials are significantly harder than pure iron. As a result, these iron alloy materials are not widely used to produce powder metallurgy articles because of their poor compressibility, often resulting in molded densities of not more than 85% of theoretical, even with the use of lubricants and binders. The low density of a powder iron alloy magnet significantly limits its magnetic properties compared to an otherwise identical magnet formed with high density pure iron. Another detrimental effect of low density is lower green strength. While sintering improves the strength of a powder metallurgy article, sintering is inappropriate for some applications, such as AC magnets that require individual powder particles to be insulated from each other with a polymeric coating, and permanent magnets that cannot withstand the high temperatures required for sintering.
In view of the above, it would be desirable if a method were available that enabled hard, lower-compressible materials to be used to produce powder metallurgy articles, and particularly hard alloy iron materials to produce powder metallurgy magnets that exhibit magnetic properties superior to pure iron powder metallurgy magnets.